Common channel Signalling System number 7, defined by CCITT Recommendations, provides support for large numbers of applications in telecommunication networks. In particular, the SS7 protocols permits systems to be optimized for operation in digital telecommunications networks in conjunction with stored program controlled exchanges.
Detailed information related to the SS7 protocol can be found in CCITT Recommendations Q. 700 to Q. 716, entitled "Specifications of signalling system number 7".
An overview of Signalling System number 7 can be found in an article "An overview of Signalling System number 7", by Abdi R. Modarressi et al, Proceedings of the IEEE, vol. 80, no. 4, April 1992, page 590.
More details can be found in a document "Common Channel Signalling System Number 7 for ISDN and Intelligent Networks" by Bijan Jabbari, Proceedings of the Generally speaking, the CCITT SS7 specification is based on circuit-related telephony control requirements. To meet these requirements, four functional levels were specified as illustrated in FIG. 1, the Message Transfer Part comprising levels 1-3 and the User part comprising level 4.
Level 1 of the Message Transfer Part defines the physical, electrical and functional characteristics of a signalling data link and the means to access it. Level 2 defines functions and procedures relating to the transfer of signalling messages over an individual signalling data link. Level 3, in principle, defines transport functions and procedures that are common to and independent of the operation of individual signalling links. Generally speaking, these transport functions fall into two major categories:
a) Signalling message handling functions that, upon actual transfer of the message, direct the message to the proper signalling link or User part. PA1 b) Signalling network management functions that, on the basis on predetermined data and information about the status of the signalling network, control current message routing and the configuration of the signalling network facilities. In the event of changes in status, the network management functions also control reconfigurations and other actions needed to preserve or restore normal message transfer capability. The detailed requirements for signalling network functions are given in Recommendation Q.704.
Reconfiguration of message routing involves the use of two particular procedures: a change-over procedure and a corresponding change-back procedure. The objective of the change-over procedure is to ensure that signalling traffic normally carried by a link that becomes unavailable is reassigned to alternative signalling link(s) as quickly as possible while avoiding message loss, duplication or mis-sequencing. For this purpose, the change-over procedure includes buffer updating and retrieval, which are performed before opening alternative signalling link(s) to the reassigned traffic. Buffer updating consists of identifying all messages in the retransmission buffer of the failed signalling link which have not been received by the far end. The identification is the result of a hand-shake procedure, based on change-over messages, performed over signalling links which continue to connect the two points at the ends of the failed signalling link.
Conversely, the change-back procedure is used to restore, to an original signalling link, traffic which had been diverted to alternative signalling links, while avoiding message loss, duplication or mis-sequencing. For this purpose, change-back includes a procedure to control the message sequence. Change-back procedures perform functions which are generally opposite to functions performed during change-over procedures.
In order to initiate a change-over procedure, it is necessary to have a mechanism to determine the signalling links which will be used to carry the traffic diverted from an original signalling link that has become unavailable. Conversely, the change-back procedure requires that a mechanism be provided for determining which traffic carried on alternative signalling links will be restored to an original signalling link that has become available again.
A known method for identifying alternative links is to designate a specific backup link for each original link. If an original link fails, all traffic which would have been carried on that link is diverted to the designated backup link. This simple process has the drawback of abruptly and perhaps heavily loading the designated backup link when the original link fails.